Tropism Notes

Tropism Notes A. What are we talking about? 1. Tropism - directional growth of a plant, or part of a plant, in response to an external stimulus a) Stimulus – an action or condition that causes a response b) Response – an action or condition that is a reaction to a stimulus 1) Negative response – when the growth of a plant is away from the stimulus 2) Positive response – when the growth of the plant is towards the stimulus B. Types of Tropism 1. Hydrotropism a) Stimulus – water b) Response – a plant’s roots grow towards a water source c) What type of response is this? Positive 2. Thermotropism a) Stimulus – temperature b) Response – a plant grows in the direction of/away from a source of heat or cold 1) Example: In the picture, this tree has a positive response to the cold. 2) Example: A plant called a Rhododendron curls its leaves away when it experiences cold weather. 3. Gravitropism a) Stimulus – gravity b) Response – 1) Negative – the stalk grows upwards, against gravity 2) Positive – the roots grow downwards, towards gravity 4. Thigmotropism a) Stimulus – touch b) Response – the leaves or part of the plant that is touched move or coil away from the touch, or trigger some other action 1) Example: The Venus Fly Trap is a plant that shows thigmotropism. It closes when a bug or some other object touches it. 5. Phototropism a) Stimulus – light b) Response – the plant grows in the direction of a unidirectional light (it only comes from one direction) 1) What type of response is this? Positive 6. Heliotropism a) Stimulus – sunlight b) Response – a plant grows or turns to face the direction of the sun 1) Example: Young Sunflowers face the sun all day long. As the sun goes across the sky, the sunflower turns so that it is always facing it. 2) Is a Sunflower growing towards the sun an example of Phototropism or Heliotropism? Both